Polyolefin microporous membranes are used as battery separators in lithium primary batteries and secondary batteries, lithium polymer batteries, lithium-hydrogen batteries, lithium-cadmium batteries, silver-zinc secondary batteries, and the like, for example. A battery separator prevents short-circuiting due to contact between the active materials of both poles and forms an ion conduction pathway by holding an electrolyte solution in the pores thereof. A battery separator thus fulfills an important function from the perspectives of battery safety and battery performance (capacity, output characteristics, cycle life, or the like). Therefore, there is a demand for polyolefin porous membranes to have excellent permeability, mechanical characteristics, impedance characteristics, and the like.
For example, an aprotic electrolyte battery separator including the lamination of a polyethylene porous membrane and a polypropylene porous membrane is disclosed in Patent Document 1, and an aprotic electrolyte battery separator having a thickness of 25 μm, a maximum pore size of 0.171 μm when measured with a mercury porosimeter, a porosity of 49.5%, and a gas permeability of 1,030 (L/min·m2·kgf/cm2) is described in a working example thereof.
In addition, a multi-layer porous film comprising a porous membrane layer (layer A) containing a polyolefin resin composition (a) having a crystal melting peak temperature of not lower than 150° C. and not higher than 250° C. as a main component and having a thickness of not less than 10 μm and a nonwoven fabric layer (layer B) containing a polyolefin resin composition (b) having a crystal melting peak temperature of not lower than 100° C. and lower than 150° C. as a main component and having a fiber diameter of not greater than 1 μm is disclosed in Patent Document 2, and a film having an air permeability of from 433 to 573 sec/100 mL and a thickness of from 22 to 49 μm is described in a working example thereof.
Furthermore, a battery separator having a porous layer containing a polyolefin resin as a main component is described in Patent Document 3, and a separator having an arithmetic average roughness Ra of not less than 0.3 μm on at least one surface of the separator, an average peak-valley roughness spacing Sm of not less than 1.3 μm on at least one surface of the separator, a bubble point pore size of from 0.02 to 0.04 μm, a Gurley value (air permeability) of from 300 to 540 sec/100 mL, a thickness of from 23 to 29 μm, and a pin puncture strength of from 2.0 to 2.9 N is described in a working example thereof.
In addition, a battery separator having a porous layer containing a polyolefin resin as a main component is described in Patent Document 4, and a separator having an arithmetic average roughness Ra of from 0.46 to 0.88 μm on at least one surface of the separator, a bubble point pore size of from 0.02 to 0.04 μm, a Gurley value (air permeability) of from 330 to 600 sec/100 mL, a thickness of from 22 to 31 μm, and a piercing strength of from 2.2 to 3.1 N (224.4 to 316.3 gf) is described in a working example thereof.
Due to demands for thinner profiles in separators in response to increases in energy density and miniaturization of lithium ion rechargeable batteries in recent years, the polyolefin microporous membranes disclosed in Patent Documents 1 to 4 all have a thickness of not less than 20 μm. Therefore, there is a demand for the development of a polyolefin multilayer microporous membrane having sufficient permeability and mechanical strength even when formed into a thin film. In addition, although the impedance of a polyolefin multilayer microporous membrane ordinarily tends to decrease due to a reduction in thickness, there is a demand for improvements to achieve a higher level of impedance characteristics.